The lost foam casting process may be utilized to cast complex metal articles, such as engine blocks. It is well documented that the lost foam casting process is an efficient and effective casting process for forming such articles. See U.S. Pat. Nos. 4,854,368; 5,014,764; 5,058,653; 5,088,544; 5,161,595 and 5,960,851.
In lost foam or evaporable foam casting, a pattern is produced from a polymeric foam material, such as polystyrene, and has a configuration identical to the metal article to be cast. A porous ceramic coating is applied to the outer surface of the pattern. One or more patterns are located within an outer vessel and a polymeric foam gating system connects each pattern to a sprue. The space between the patterns and the vessel is filled with a finely divided inert material, such as sand, and the sand also fills the internal cavities within the pattern.
In the lost foam casting process, molten metal is fed into the sprue and the heat of the molten metal acts to decompose or ablate the polymeric foam material of the gating system, as well as the pattern. The molten metal occupies the void created by ablation of the foam material, with the decomposition products of the foam passing through the porous ceramic coating of the pattern and becoming trapped within the interstices of the sand. Upon solidification of the molten metal, the resulting cast article has a configuration identical to the polymeric foam pattern.
One of the advantages of the lost foam casting process is that it is capable of forming complex internal passageways during casting, such as the internal passageways of an internal combustion engine. However, a significant disadvantage of the lost foam casting process is the difficulty of removal of the residual ceramic coating from the complex internal passageways of the casting. For this reason, lost foam casting is not used to a significant extent in the production of engine blocks because the presence of the residual ceramic coating adversely affects the performance of running engines.
As with most cast articles, it is desirable to heat treat articles after they are cast using the lost foam casting process. In general, heat treating is the process by which an alloy is elevated to a high temperature, thereby changing its microstructure to improve its properties. Through this thermal treatment, the resulting properties and performance of a component may be manipulated. Specifically, when dealing with aluminum silicon alloys, heat treatment changes the alloy's microstructure by spherodizing and coarsening eutectic silicon particles, and homogeneously redistributing precipitate forming elements in solid solution. It is known in the art that the heat-up rate and the time spent at the heat treatment temperature are important factors in obtaining the properties which will increase performance of a heat treated article.
Short term solution heat treating has been advanced by Franz Feikus in his 1998 paper published in Light Metal Age entitled “Heat Treatment of Aluminum Casting Alloys for Vacuum Die Casting,” by Liss Pederson's book “Solution Heat Treatment of AlSiMg Foundry Alloys,” and by Ray Donahue's 2001 paper delivered for the Worchester Polytechnic Institute Heat Treating Conference entitled “Short Term Solution Heat Treating—The Industrial Application.” These publications indicate that short term solution heat treating provides the advantages of heat treatment with a minimal amount of time. However, because conventional short term heat treating processes utilize an air circulating furnace, a significant amount of time (up to 3.5 hours) is forfeited in heating the cast article to the required heat treatment temperature. Thus, conventional short term heat treatment processes do not effectively take advantage of the time that could be saved.
In accordance with the invention, the use of a heated, fluidized sand bed has been found to economically heat treat articles cast using the lost foam casting process while simultaneously efficiently and effectively removing the residual ceramic coating remaining from the lost foam casting process. The economy and efficiency of the invention is enhanced through the use of the heated, fluidized sand bed by significantly shortening the time for complex cast metal articles to reach appropriate heat treatment temperatures. The heated, fluidized sand bed contemplated for use in the present invention is described in U.S. Pat. No. 6,042,369, which is incorporated herein. The fluidized heat treatment beds, as described therein are very accurate, in so far as they deviate very little from the desired heat treatment temperature. This further enhances the efficiency of the treatment process. Additionally, when the heated, fluidized sand bed is utilized in conjunction with the lost foam casting process, further production efficiencies may be realized.
Unexpectedly, the fluidized action of the sand in heated fluidized sand beds has been found to clean the residual ceramic coating from complex cast articles to a degree that could not be realized with prior cleaning methods. Particularly, the inside passageways of complex castings, such as engine blocks, are now easily and sufficiently cleaned.
It has also been realized that the cleaning action of the heated, fluidized sand bed may be optimized by incorporating specifically shaped media. Tetrahedral shaped media has been found to optimize the cleaning action of the heated fluidized sand bed so that the cleaning time is not significantly longer than the optimized solution heat treating time.
Other factors also contribute to the excellent cleaning action of the present invention. Thermal shocking of the ceramic coating microcracks the coating aiding in its removal. The thermal shock propensity of a material is directly proportional to its thermal expansion coefficient as well as the temperature difference between the material in the bed and the atmospheric temperature. Further, the thermal shock propensity is inversely proportional to the material's thermal conductivity. When heat treating in a fluidized bed, a part is directly immersed in a hot fluidized sand. The residual ceramic coating may go from as low of a temperature as room temperature to as high as 1030° F. when placed in the sand. This large change in temperature thermally shocks the ceramic thereby microcracking the coating and aiding in its removal.
Interfacial stress also helps remove the ceramic coating. When a casting with a residual ceramic coating on a surface is heated extremely rapidly in a fluidized bed, the aluminum expands at a faster rate than the ceramic coating due to its higher coefficient of thermal expansion and higher thermal conductivity. This places the ceramic in a state of tension and builds high sheer stresses at the interface between the ceramic coating and the metal. The tensile stresses in the ceramic subsequently fracture the residual coating. Thus, the high sheer stresses at the interface serve to “spall” the coating from the surface of the aluminum part.
Finally, thermal decomposition of the ceramic coating occurs at high heat treatment temperatures. The accuracy of heat treatments in a fluidized bed makes it possible to heat treat at temperatures closer to the melting point of the aluminum in the casting. The ceramic coating used to coat the forms in the lost foam casting process typically contain a percentage of a polymeric binder as an additive. The ability to solution heat treat at a higher temperature over time decomposes the polymeric binder faster than is possible at lower temperatures.
Still another production efficiency may be realized when the heated, fluidized sand bed is utilized in conjunction with a lost foam casting process. As aforementioned, a lost foam casting process uses sand or other inert sand-like material to support and surround the polymeric foam pattern, including filling any complex internal passageways. Surprisingly, it has been discovered that the same sand used in lost foam casting may be used in the heated fluidized bed to heat treat and clean the castings. After the polymeric foam pattern has been ablated, and an article is cast using the lost foam casting process, a bonded sand cluster forms in the casting flask. The bonded sand cluster is formed when the polymeric foam material enters the interstices of the sand surrounding and supporting pattern during ablation of the pattern by the molten metal. The ablated polymeric foam material subsequently solidifies in the interstices of the sand, creating a bonded sand cluster around the newly formed cast article.
Transfer of the cluster, with much of the compact sand in place, directly into the heated fluidized sand bed creates several advantages. First, because the cast article contains heat energy from the casting process, it is possible to transfer the cluster at a temperature of approximately 700° F., instead of room temperature, allowing a faster heat-up to a heat treatment temperature of approximately 1000° F., creating energy savings and cost reduction. Second, the sand used in the lost foam casting process need not be reclaimed because the fluidized beds provides in situ removal of organic deposits formed through foam decomposition. Third, introduction of the cluster into the fluidized bed creates an alternate method to capture the emissions from the shake out of the bonded sand cluster.
The inventive process also provides an advantage for alloys that can be directly quenched from above the solvus temperature to avoid the need of heat treatment. For these alloys, aging may be done in the fluidized bath, while the action of the sand in the fluidized bed removes the residual ceramic coating from the internal passageways. Further, it is possible with the present invention, to treat the casting at another temperature, which may be room temperature, or another temperature in the range between room temperature and optimal heat treating temperature for the particular metal.
Thus the present invention discloses a simultaneous method for heat treating and cleaning a cast metal article having complex passageways formed utilizing the lost foam casting process. A cast metal article using the lost foam casting process is positioned in a manner so that the fluid action of a heated fluidized sand bed easily flows through the complex passageways of the cast article. In practice, the article is attached to a fixture which holds the article in position and moves the article through the heated fluidized sand bed for a fixed amount of time. The article is then removed from the sand bed, quenched and aged to form a final cast article.
Using this process, a bonded sand cluster formed during a lost foam casting process may be directly transferred from the lost foam casting flask into the heated fluidized sand bed providing significant efficiencies in the heat treatment and cleaning of the cast article.